The present invention relates to an MR (magnetic resonance) imaging method and apparatus capable of reducing ghosts due to increasing echo signal intensity fall-off in the course of an echo train and/or due to phase errors.
The FSE (fast spin echo) method is a data acquisition method which applies m(.gtoreq.2) inversion pulses per excitation pulse to generate first to m.sup.th echoes, acquire data for m views in a k-space from the echoes, and repeats these steps M/m times, where M is the total number of views in the k-space (the total number of data sets in the phase axis direction), to acquire data for all views in the k-space.
This method can be conducted by a sequential process or a centric process depending on the way of determining which view data is acquired by which echo.
The sequential process divides a k-space into first to m.sup.th segments in the phase axis direction, assigns first to m.sup.th echoes serially to the first to m.sup.th segments, and acquires data for a certain view from an echo assigned to a segment to which the data belongs.
The centric process divides all views in a k-space into first to 2m.sup.th segments in the phase axis direction, assigns first to m.sup.th echoes serially to the first to m.sup.th segments and m.sup.th to first echoes serially to the (m+1).sup.th to 2m.sup.th segments, and, when a time period up to an .alpha..sup.th echo is an effective TE, cyclically shifts the individual segments so that the segment center of the .alpha..sup.th echo is placed in the k-space center, and acquires data for a certain view from an echo assigned to a segment to which the data belongs.
Other data acquisition methods include the SSFSE (single shot fast spin echo) method which applies M inversion pulses per excitation pulse to generate first to M.sup.th echoes and acquires data for all views in a k-space from the echoes.
FIG. 1 is a schematic diagram illustrating the signal intensity distribution of data acquired while applying phase encode based on the echo-to-view assignment in a k-space according to the sequential process.
FIG. 2 is a schematic diagram illustrating the signal intensity distribution of data acquired while applying phase encode based on the echo-to-view assignment in a k-space according to the centric process.
As shown in FIGS. 1 and 2, there are stepwise differences in signal intensity among the segments. This is caused by increasing echo intensity fall-off in the course of an echo train. This leads to the problem of the appearance of ghosts in the image.
FIG. 3 is a schematic diagram illustrating the phase error in each echo in an echo train according to the FSE or SSFSE method.
The generated phase errors have opposite directions (signs) at odd-indexed echoes and even-indexed echoes. This also leads to the problem of the appearance of ghosts in the image.